A preform made by the well-known MCVD process consists of a tube of silica glass surrounding a core which is built up by vapor deposition of successive layers of suitably doped silica on its inner wall surface. The tube, after drawing, becomes the outer sheath or cladding of the fiber.
Silica is known to have a minimum-attenuation window in a wavelength range between 0.7 and 1.6.mu., in a zone of overlapping tail ends of spectral regions of ultraviolet absorption due to electronic transitions and infrared absorption due to molecular vibrations. The utility of this range for the transmission of optical signals is, however, further restricted by two absorption peaks at the second and third harmonics of the fundamental vibration of the hydrogen-oxygen bond which corresponds to a wavelength of 2.8.mu.. This absorption is due to contamination by hydroxyl (OH.sup.-) groups in the fiber material which is particularly noticeable in fibers drawn from preforms made by the MCVD technique, owing apparently to the fact that the elevated temperatures used in the process promote the diffusion of such groups from the supporting tube into the core of the preform. This assumption is based on the observation that the concentration of various contaminants originally present in the supporting tube and in the employed reagents decreases exponentially toward the preform axis; after drawing, the same law of distribution is found to exist in the finished fiber.
The contamination by impurities (e.g. halides or oxygen) present in the reagents can be reduced by various purification methods such as distillation, sublimation, filtering or the use of photochemically activated chlorine. As far as we are aware, however, there has not yet been found a satisfactory solution to the problem of contamination of the preform core by OH.sup.- groups released from its supporting tube. Thus, for example, the interposition of a barrier layer of low-viscosity compounds between the inner tube wall and the first chemically desposited core layer has only a limited effect in reducing the contamination.